1. Field of the Invention
The present invention relates to a clip for clipping sheets of paper, and particularly relates to a clip in which end portions abutting on each other are opened by a pair of finger grips to clip sheets of paper between these end portions.
2. Description of the Related Art
To clip loose sheets of paper, there has been used a clip called a double clip widely. This clip is constituted by a body made from a band-like elastic plate such as steel and a pair of finger grips attached to the body. The body has a back portion formed in a longitudinally central portion of the band-like metal plate so as to have a length substantially corresponding to a maximum clipping thickness of the clip, and abutment portions formed by bending the band-like metal plate in the same direction at the lengthwise opposite ends of the back portion so that the respective free ends of the abutment portions abut against each other elastically. The pair of finger grips are formed from hard metal wire, and the opposite ends of each of the finger grips are rotatably inserted into ring-like lock portions formed by bending the free end portion of each of the abutment portions. To clip a stack of sheets of paper or documents, one may grip and rotate the free ends of the two finger grips by fingers toward the back portion, that is, in the direction to make the free ends of the finger grips come close to thereby spread the nip between the respective free ends of the abutment portions to clip documents or the like in the nip.
Hereinafter, with respective to a clip, the term "length" means a size in the longitudinal direction of the band-like metal plate from which the clip is produced and the term "width" means a size in the widthwise direction perpendicular to the longitudinal direction of the band-like metal plate.
In such a clip, essential portions for clipping documents or the like are the free ends, that is, the abutting edges of the two abutment portions of the body. The width or widthwise size of each of the abutting edges corresponds to the width of the elastic metal plate. The elastic force acting on the abutting edges of the two abutment portions to make the abutting edges come close to each other is produced by bending the band-like metal plate at the borders between the back portion and the two abutment portions to make the abutting edges of the abutment portions abut against each other elastically. This clip body, when viewed from a side, forms an isosceles triangle having a base formed of the length of the back portion, and other two sides which are equal in length and formed of the abutment portions. In this case, if the band-like metal plate are bent at the two borders so that the abutting edges of the abutment portions abut against each other simply, the abutting edges of the abutment portions cannot abut against each other completely all over their width surely and the clip cannot clip a few thin sheets of paper firmly. Further, a clipping force is so weak that the clip cannot clip thick sheets of paper. Therefore, the back portion is curved inward and extended toward its ends along its width to thereby make the interior angles of the respective borders acute. With the interior angles made acute, a virtual intersection point at the abutting edges of the two abutment portions moves to a center portion of the triangle, so that the abutting edges of the abutment portions abut against each other tightly. Accordingly, the clip can clip thin sheets of paper. In addition, since the clipping force increases, the clip can clip thick sheets of paper surely. Therefore, although the normal isosceles triangle is formed so that the two interior angles at the borders are about 65.degree. respectively by way of example, if the back portion is curved inward as mentioned above, the interior angles take, for example, about 40.degree. to thereby generate a strong elastic force between the abutting edges of the two abutment portions. Although the value of these interior angles depends on the size of the clip, needless to say, it is necessary to make the two bending angles, that is, interior angles at the borders be not larger than 90.degree..
On the other hand, for example, JP-U-6-79580 and JP-U-7-31374 disclose a so-called corner clip in which an opening is formed substantially in a widthwise center portion of a back portion, so that corner parts of sheets of paper are be clipped is made to project outside through this opening so as to make the clip easily clip the corner part of the sheets of paper. Further, JP-U-8-28566 discloses a modification of the corner clip disclosed in the above JP-U-7-31374, in which the lengthwise edge of the opening is limited up to the borders between the back portion and the abutment portions to thereby prevent the abutting edges of the abutment portions from being loosened by the formed opening.
In the conventional clips, as mentioned above, the elastic force for making the abutting edges of the abutment portions abut against each other is generated by bending the band-like metal plate at the two borders between the back portion and the abutment portions. Further, the back portion is curved inward to make the interior angles of these bent portions smaller to thereby increase the elastic force of the respective abutment portions against each other. As a result, the elastic force having a value which is stronger than the required value is generated between the abutting edges of the abutment portions in the direction to make the abutting edges close to each other elastically. Accordingly, it is difficult to open the clip enough so that the abutment portions become parallel with each other when a stack of sheets of paper having a thickness which is near an allowable maximum thickness of the clip is clipped, and particularly in a large clip, a very strong force is required to open the clip. In addition, when the sheets of paper are once clipped, a strong clipping force is applied to the sheets of paper so that the sheets of paper are not removed from the clip even if the clipped sheets of paper are lifted up from the clip to give a strong shock to the sheets of paper at the clip. Accordingly, there is a case that stripe marks caused by the abutting edges of the abutment portions are left in the sheets of paper after the clip is removed from the sheets of paper.
However, when the clipped sheets of paper are to be handled, there is few case in which while the finger grips are gripped by fingers to handle the sheets, a strong shock is given to the sheets. Accordingly, it is requested to make the clipping force weaker so that the abutting edges of the abutment portions can be opened easily by a weaker force. The acuter the interior angles are, the stronger the elastic force becomes. Therefore, in this case, the elastic force can be reduced if the interior angles at the borders are increased. However, the abutting edges of the abutment portions are opened up to the interior angle or more when the abutting edges of the abutment portions are opened. When this operation is repeated, setting is caused at the bent portions, so that there is a fear that a gap is produced between the abutting edges of the abutment portions when the clip is opened after clipping thick sheets of paper for a long time, and thin sheets of paper may be detached easily when such thin sheets of paper are clipped next. In addition, in the corner clip disclosed in the above-mentioned JP-U-6-79580 or JP-U-7-31374, in which an opening is provided in a back portion, the elastic force is not reduced if the opening is provided only in the back portion. On the contrary, the above JP-U-8-25866 states that the elastic force is reduced corresponding to the opening if the borders between the back portion and the abutment portions are partly deleted by the opening, and the strength of the clip body is reduced and easily broken if the opening is further extended into the abutment portions. JP-U-8-25866 further states that in order to prevent this, therefore, it is necessary to limit the opening up to the borders between the back portion and the abutment portions. Accordingly, the opening in the back portion is only for making corner parts of sheets of paper project out therefrom, without any intention to reduce the elastic force by the effect of the opening so as to easily open the clip to make the abutment portions parallel with each other.